A method of filling in the white line artifact that appears at the boundary between two printed regions of an image, where the two printed areas have a different halftone frequency specification, by identifying the fast scan coordinate of the single pixel where the white area is produced and filling it in with a high frequency pixel using a color equal to the lower frequency color. In digital printers, a higher frequency halftone pattern is typically used for text for greater edge enhancement while a lower frequency halftone pattern is used to enable a greater stability of color rendering. The Digital Front End (DFE) or raster processor determines which of the two should be used, based on object type or other specified rendering characteristics. For example, a high xe2x80x9ctagxe2x80x9d on the data would indicate that text is being supplied to the printer so the high frequency pixels should be used, and a low xe2x80x9ctagxe2x80x9d on the data would indicate low frequency pixels for constant color or color pictures. To preserve the best possible text edge, the system must be capable of switching at the boundary between the high frequency and low frequency screens. This leads to a possible phase problem in that for some locations both the high and low frequency screen patterns can apply toner, whereas for others only the high frequency screen can apply toner. If the DFE specifies switching from high to low (or vice-versa) at the beginning of a pixel location that can not be printed with the low frequency halftone screen, no toner will be applied, and an undesirable white space is produced. This is especially noticeable if both bordering colors are dark. A remedy is needed which fills the space with the correct color while still preserving an accurate text edge.
This invention addresses this problem by modifying the data while in the run length encoded format (before the data is sent to the printing device). In this data description, a run consists of a contiguous set of pixels along a single raster line (scan line) with similar attributes. It is typically described by a color, start position, and length, as well as any rendering attributes (such as which halftone screen should be used). In this sense, object edges in the fast scan direction are easily determined, since abutting runs are assumed to have different attributes. The method involves inspecting the halftone screen specification of abutting runs in the fast scan direction. If a screen boundary exists at the interface, the location of the boundary pixel of the low frequency run is inspected to determine whether or not toner will actually be applied at that pixel. If the location of this low-frequency boundary pixel is found to be xe2x80x9cout of phasexe2x80x9d with respect to the halftone screen pattern of the printing device (i.e. the screen does not apply toner at this location), the interface will be modified; otherwise it will be untouched. Modification is a two step procedure, involving first the reduction in length of the low frequency run at the interface by one pixel, and second inserting a new single pixel run at that location with a color equal to that of the xe2x80x9cdeletedxe2x80x9d pixel, while using the high frequency halftone screen. This ensures that the objectionable white gap at object/screen interfaces will be eliminated.